Hybrid vehicles operate by power from an internal combustion engine and power from a motor through a battery. In particular, hybrid vehicles are designed to efficiently combine and use the power from the internal combustion engine and the motor.
For example, as illustrated in FIG. 1, a hybrid vehicle includes: an engine 10; a motor 20; an engine clutch 30 which controls power transmission between the engine 10 and the motor 20; a transmission 40; a differential gear 50; a battery 60; an integrated starter-generator (ISG) 70 which starts the engine 10 or generates electric power by output of the engine 10; and wheels 80.
As further shown, the hybrid vehicle includes: a hybrid control unit (HCU) 200 which controls overall operation of the hybrid vehicle; an engine control unit (ECU) 110 which controls operation of the engine 10; a motor control unit (MCU) 120 which controls operation of the motor 20; a transmission control unit (TCU) 140 which controls operation of the transmission 40; and a battery control unit (BCU) 160 which manages and controls the battery 60.
The battery control unit 160 may also be referred to as a battery management system (BMS), and the integrated starter-generator 70 may also be referred to as a starting/generating motor or a hybrid starter & generator.
The hybrid vehicle may operate in an electric vehicle (EV) mode using only power of the motor 20, a hybrid electric vehicle (HEV) mode using torque of the engine 10 as main power and torque of the motor 20 as auxiliary power, or a regenerative braking (RB) mode during braking or when the vehicle runs by inertia. In the RB mode, braking and inertia energy are collected through power generation of the motor 20, and the battery 60 is charged with collected energy.
Since the hybrid vehicle uses both an engine and a motor, the hybrid vehicle needs to perform different control of limiting maximum speed (e.g., 6500 rpm) from a vehicle using only an internal combustion engine as a main power source.
Controlling the maximum speed limit is performed to protect a power source and a system of a vehicle when a vehicle's maximum output is required.
The vehicle using the internal combustion engine as the only power source may control processes of the maximum speed limit as follows.
E—1: An engine control unit (ECU) controls an engine to output a torque corresponding to an engine speed according to the output characteristic of the engine.
E—2: Since the engine speed is equal to speed of an input shaft of a transmission, a transmission control unit (TCU) performs shift-speed control so that the engine speed may not exceed the maximum speed.
E—3: When the engine speed reaches the maximum speed due to shift-changing delay at process E—2, the ECU limits output of the engine to 0 torque by performing fuel-cut control.
Hybrid vehicles generally run in the HEV mode since the maximum power of the engine and the motor is required in a high rpm range for the hybrid vehicles when high power is requested.
In the HEV mode, the engine clutch of the hybrid vehicle is in a lock-up state in order to transmit engine torque to a driving shaft. In the lock-up state of the engine clutch, the speed of the engine is synchronized with the speed of the motor.
As described above, since the hybrid vehicle uses two power sources, the internal combustion engine and the motor by a battery, controlling a maximum speed limit of the hybrid vehicle should further comprise a function to limit maximum speed related to the motor, unlike the vehicle using solely an internal combustion engine as the main power source.
General control processes for the limiting maximum speed of the hybrid vehicle may be described as follows.
H—1: The HCU controls the engine and the motor with the ECU and the MCU so that the engine torque and motor torque corresponding to the engine speed and motor speed may be outputted according to an output characteristic map of the engine and an output characteristic map of the motor.
H—2: The TCU performs shift-speed control so that the engine speed and the motor speed may exceed the maximum speed, respectively.
H—3: When the engine speed and the motor speed reach the maximum speed due to shift-changing delay at process H—2, the HCU limits output from the engine and the motor so that each of the engine and the motor may output 0 torque by feedback control based on a map table.
H—4: As shown in FIG. 2, when the engine speed and the motor speed increase despite performing process H—3, each of the ECU and the MCU controls a limiting of output to 0 torque by itself.
However, as described above, since the control method of limiting the maximum speed of the engine and motor of the hybrid vehicle performs feedback control based on the map table, problems may occur as follows.
FIG. 3 is an exemplary graph for explaining problems of a control method of limiting the maximum speed of an engine and motor of a hybrid vehicle according to the related art.
Referring to FIG. 3, since the control method of limiting the maximum speed of the engine and motor of the hybrid vehicle according to the related art sets maximum output torque according to a 2-dimentional map table based on speed of the engine and motor, torque variation may badly occur.
That is, the control method of limiting the maximum speed of the engine and motor of the hybrid vehicle according to the related art may cause problems as follows.
First, since output torque is limited according to the speed of the engine and motor, a value of limiting output torque of the engine and motor may be badly varied while input speed of the transmission is varied. For example, referring to FIG. 3, when input speed of the transmission is varied from 6000 rpm to 6200 rpm, output torque may be varied to maximum of 500 N·m (=200 N·m+300 N·m). In this case, a judder or shock may occur in the hybrid vehicle.
Second, it may be difficult to keep the input speed of the transmission at a speed value for limiting the maximum speed of the engine or motor. That is, when the maximum speed of the engine or motor is limited to 6200 rpm, input speed of the transmission may exceed 6200 rpm. In this case, protecting the transmission hardware system and the electrical-power-device hardware system may be difficult.
Third, when the engine clutch is in a lock-up state, maintaining a consistent limiting maximum torque value of the engine and the motor is difficult due to speed difference between the engine and the motor. For example, when the engine is in a fuel-cut state due to a maximum torque limit, because the motor generates additional torque in order to satisfy demand torque, a state of charge (SOC) of the battery may deteriorate.
Fourth, a shift shock and shift delay may occur because of a bad variation of input torque of the transmission.
The above information disclosed in this section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.